1. Field of the Invention
The present invention relates to Class D electronic amplifiers, and in particular, to amplifier output stage over-current protection circuits.
2. Discussion of the Related Art
In a Class D amplifier, a pulse-width modulator (PWM) converts an incoming signal into a high-frequency rectangular wave, the average value of which tracks the incoming signal magnitude. This rectangular wave is fed to a set of power FET's, which provide an output signal that is filtered and then used to drive an external speaker. As shown in FIG. 1, a signal ANALOG.sub.-- IN is received by a pulse-width modulator, or PWM 101. PWM 101 generates a signal PWM.sub.-- OUT, which is a rectangular wave of a frequency defined by a PWM clock CK101. Signal PWM.sub.-- OUT is then split and fed through level shifter/line driver circuits 103, 104, 105 and 106, to FET's Q107, Q108, Q109 and Q110, respectively, which make up an FET output bridge 121. An inductor L117, a capacitor C119, and an inductor L118 provide output filtering that averages the digital output of the output bridge and provides an analog driving signal across external speaker 120. Speaker 120 can be a single speaker or a set of speakers. When signal PWM.sub.-- OUT is in a HIGH state, circuit 103 activates FET Q107 and circuit 104 activates FET Q110. At the same time, because an inverter 102 inverts signal PWM.sub.-- OUT as it is going to circuits 104 and 105, FET's Q108 and Q109 are turned off. As a result, current flows from FET Q107 to FET Q110, through speaker 120. When signal PWM.sub.-- OUT switches to a LOW state, FET's Q107 and Q110 are turned off, and FET's Q108 and Q109 are activated. Now current flows through speaker 120 in the opposite direction, from FET Q109 to FET Q108. This alternating current flow drives the voice coil in speaker 120, reproducing the original input signal ANALOG.sub.-- IN as an audio output.
In the FET output bridge of a conventional Class D amplifier, the individual FET's have a limited current sourcing capacity. A short at the FET bridge outputs or an overly large input signal can place excessive current demands on the FET's, leading to possible damage to both the amplifier and the external load. A typical defense against this scenario is to use protective fuses, as illustrated by fuses F111 and F112 in FIG. 1. An over-current situation in FET Q107 would cause fuse F111 to blow before any major damage to the amplifier could occur, while an over-current in FET Q108 would break the link in fuse F112. In either case, however, the blown fuse would have to be replaced before the amplifier would regain functionality. Therefore, although this method provides positive protection against overload, recovery from an over-current incident is cumbersome. A second over-current protection scheme used in conventional Class D amplifiers alleviates some of the inconvenience associated with fuses. Instead of a fuse, a small resistor is placed in series with each FET. The voltage drop across the resistor is monitored, and the amplifier is shut down when the voltage drop exceeds a predefined limit. This implementation is depicted in FIG. 1 by resistors R113 and R114. Comparator circuits 115 and 116 monitor the voltage drops across resistors R113 and R114, respectively, and generate a signal SHUT.sub.-- DOWN to turn off power to the amplifier when a threshold voltage limit is passed. While this method provides a more precise over-current setpoint and easier recovery from over-current situations, the resistor in the signal path increases power consumption during normal operation, decreasing overall amplifier efficiency. In addition, both of the described methods are a less-than-optimal solution to the scenario in which the excessive currents are caused by a large incoming signal. While a short-circuit definitely requires some sort of immediate examination, a large incoming signal may only be temporary, and is not necessarily indicative of a severe problem. However, conventional protection schemes preclude continuous amplifier output when any over-current situation arises.
Accordingly, it is desirable to provide an over-current protection circuit that consumes low quiescent power and maximizes amplifier output while still preventing damage from short-circuit situations.